A variety of metals having significant industrial uses are not found naturally in their elemental forms. Rather, these metals are mined as a variety of compounds from which the desirable metal product must be extracted. One such metal is aluminum. Commercially, aluminum is produced from naturally occurring aluminum compounds by the electrolytic reduction of alumina Al.sub.2 O.sub.3. Alumina is obtained from bauxite ore by the Bayer process which involves digesting crushed bauxite ore in strong caustic soda solution.
In 1886, Charles Hall in the United States and Paul Heroult in France independently developed the currently employed electrolytic process for extracting aluminum from alumina. This process, known today as the Hall-Heroult process, transformed aluminum from a precious metal into a common structural material. The process is still the most widely used commercial process for obtaining aluminum metal and is fundamentally the same as it was originally disclosed by Hall and Heroult in 1886.
In the Hall-Heroult process, electric current is passed through molten electrolyte containing alumina. An important feature of the Hall-Heroult discovery was that cryolite, a double salt of aluminum and sodium, represented by the chemical formula, Na.sub.3 AlF.sub.6, would dissolve alumina at temperatures around 1000.degree. C. and that the dissolved alumina could be electrolytically reduced to form molten aluminum metal.
The electrolytic reduction of metals is often performed in large cells or pots. These cells typically have massive carbon cathodes at the base and carbon anodes, normally formed in the shape of large blocks, suspended above the cell and capable of being lowered into the electrolyte. Direct electric current is passed from the anodes through the electrolyte to the carbon cathodes. During the reduction of alumina, for example, the carbon anodes are consumed in the chemical reaction occurring in the cell. This reaction can be represented, as follows: EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.4Al+3CO.sub.2.
This process yields an aluminum product that is very pure, e.g., 99.0% to 99.8%. The main impurities are traces of iron and silicon.
Despite its capability to produce high purity aluminum, the Hall-Heroult process has always suffered a number of significant problems. The most important of these arises from the use of consumable carbon anodes. These anodes are expensive to produce, and this cost adds significantly to the overall cost of aluminum produced by the Hall-Heroult process. Furthermore, it is difficult to maintain uniform anode current loading during use since the anodes are consumed resulting in a continuous change in their shape.
Because of the problems associated with carbon anodes, substantial research has been conducted in an effort to find another anode material, particularly what has been referred to as a non-consumable anode. Such an anode does not react with oxygen formed at the anode, does not dissolve in the electrolyte and is not consumed in the electrolytic reaction. Unfortunately, the research conducted to date has not resulted in the development of a fully satisfactory anode material.
Another set of problems with electrolytic reduction cells arises from the use of a carbon lining in the cell. These cells are operated under conditions that cause molten electrolyte to freeze on the sidewalls of cells during operation so that molten electrolyte floating on molten metal product is contained within a shell of frozen electrolyte. This is necessary to prevent reaction between the carbon cell lining and constituents of the molten electrolyte during operation of these cells. The position of the interface between the frozen and molten electrolyte changes, however, during operation of cells, making it difficult to establish uniform operating conditions. Also, the maintenance of frozen electrolyte at the sidewalls results in a substantial heat loss from the cell which in turn reduces its thermal efficiency.
A third set of problems associated with electrolytic reduction cells arises from the lack of a suitable cathode material. Presently, carbon is used as the cathode material in these cells. Unfortunately, a product such as molten aluminum does not wet carbon. Therefore, in the case of aluminum production, it is necessary to maintain a deep pool of molten aluminum on the bottom of the cell. This is required because the carbon cathode surface must be fully covered in order to prevent contact between the molten salt electrolyte and the cathode itself in the presence of molten aluminum. Otherwise, the formation of aluminum carbide occurs, and this both reduces the productivity of the cell and consumes the carbon cathode.
The presence of the deep pool, however, creates a new problem. The cell currents are generally extremely high, typically on the order of about 100 kA to about 300 kA. At such currents, electromagnetic forces can cause the molten aluminum to develop waves of substantial physical dimension. To prevent electrical shorting of the molten aluminum to the anode, allowance must be made in the separation of the anode and cathode. This results in an excessive voltage drop across the electrolyte and contributes to poor energy utilization within the cells.
Problems such as those discussed above for Hall-Heroult cells also exist with other electrolytic cells and processes for the electrolytic production of metals from oxide based feed materials. This has in many instances, resulted in the metals being produced from more expensive feed materials or by use of more complicated and expensive processes than would be required if oxide based feed materials could be used.
As such, a need exists for electrodes and cell linings in electrolytic cells used for the reduction of oxide based feed materials that are not consumed under the operating conditions of the cell, allow closer anode/cathode spacing, and can be shaped to electrode and cell lining configurations that are thermally and mechanically stable.